Expose the underlying Mongo driver in RealMongo.

This makes the underlying Mongo instance in RealMongo publicly
accessible.
Fixes a bug where parsing from a file could halt early.
Bump for M6.3 release
Fix JParser to better respect ByteBuffer's position/limit.
Version bump for M6.4 release.
Cherry-pick commit d4b35115 to get parseUnsafe.
Merge pull request #82 from non/milestones/1.0.0-M6.x

Cherry-pick commit d4b35115 to get parseUnsafe.
Bump version to M6.4
Merge remote-tracking branch 'jdegoes/milestones/1.0.0-M6.x' into milestones/1.0.0-M6.x
Don't complete ChunkHandler promise with failures on normal close/disconnect

channelClosed and channelDisconnected get called for all close/disconnect
events, whether under normal ops or in error, so we shouldn't mark the
promise as a failure if we've already marked it as a success.